Metal oxide silicon field effect transistors (MOSFETs) are high impedance,,low current devices, and thus are particularly susceptible to damage by electrostatic discharges. Electrostatic discharges ranging from a few volts to many thousands of volts can be coupled to a packaged integrated circuit by simply handling the devices. The discharge can be transferred through either the input or output terminals of the device. The output terminals of an integrated circuit are particularly susceptible to damage from electrostatic discharge, as such outputs are characterized by a low secondary breakdown voltage. The inputs of a MOSFET integrated circuit cannot withstand high input voltages because of the low voltage breakdown characteristics of the thin gate insulator material.
Conventional field effect transistors comprising the outputs of an integrated circuit are typically constructed with spaced-apart N-type drain and source regions formed in the face of a semiconductor material. The gate insulator and conductor bridge the regions on top surface of the semiconductor material. Metallic source and drain conductors, generally formed of aluminum, typically make contact directly to the respective semiconductor regions. The bulk resistance of each semiconductor region has heretofore provided the only means of limiting large currents resulting from electrostatic discharges. The bulk resistivity of the source and drain regions are somewhat effective in limiting electrostatic discharge currents, and represent an economical and easily fabricated mechanism for protection against the effects of electrostatic discharges. In some instances, the metallic conductors are spaced apart as much as possible from the edge of the polysilicon to thereby increase the bulk resistance between the conductor and the junction.
However, even though electrostatic currents can be limited to safe magnitudes, semiconductor region imperfections can often form centers or areas of high current densities. The high current density can heat the semiconductor area to its eutectic temperature and form a melted filament, thus destroying the device. The uncontrolled heating of semiconductor materials is also detrimental as thermal runaway may result. That is, due to the negative temperature coefficient of semiconductor material, an increase in temperature reduces its bulk resistance and it can then support an increased current flow. It is apparent that high temperatures resulting from large electrostatic currents can cause the self destruction of the device.
With the trend toward high density integrated circuit chips, the foregoing electrostatic protection technique has been unintentionally obviated in favor of other advantages. Particularly, many MOSFET transistors used as output devices of integrated circuits include aluminum contacts formed on silicide-covered source and drain regions. The silicide covering is highly conductive so that other circuit connections can be easily formed in contact with the respective source or drain regions of the transistor. However, since the electrostatic discharge current will then flow through the low resistance of the silicide layer, the bulk resistivity of the source and drain semiconductor regions can no longer provide the mechanism for limiting discharge current. Therefore, the MOSFET transistor is left essentially with no electrostatic discharge protection. Moreover, the aluminum material has a much lower melting point than that of the underlying semiconductor region, and thus high electrostatic discharge currents can melt the aluminum material, causing it to diffuse through the thin semiconductor region and short circuit the junction.
From the foregoing, it can be seen that a need exists for a method and structure for reliably assuring that a MOSFET transistor is protected against damage from high voltage electrostatic discharges. There is an associated need for electrostatic discharge protection used in connection with high current output MOSFET devices which are typically characterized by low breakdown voltages.